Conventionally, a thin film such as silicon oxide (SiO2) or silicon nitride (Si3N4 or the like) is widely used in a semiconductor element such as a thin film transistor, a photoelectric converting element or the like. The following three kinds of techniques for forming a thin film such as silicon oxide, silicon nitride or the like are mainly used.
(1) Physical vapor-phase film forming technique such as sputtering or vacuum deposition
more specifically, a technique for converting a solid thin film material to an atom or an atomic group to be physical means and depositing the same on a surface of a formed film, thereby forming a thin film;
(2) Thermal CVD technique
more specifically, a technique for heating a gaseous thin film material to a high temperature so as to induce chemical reaction, thereby forming a thin film; and
(3) Plasma CVD technique
more specifically, a technique for converting a gaseous thin film material to a plasma so as to induce chemical reaction, thereby forming a thin film.
In particular, the plasma CVD technique (plasma enhanced chemical vapor deposition) in (3) is widely used because a minute and uniform thin film can efficiently be formed.
A plasma CVD apparatus 100 to be used in the plasma CVD technique is generally constituted as shown in FIG. 4.
More specifically, the plasma CVD apparatus 100 comprises a reaction chamber 102 maintained in a decompression state, and an upper electrode 104 and a lower electrode 106 are provided to be opposed apart from each other at a constant interval in the reaction chamber 102. A film forming gas supply path 108 connected to a film forming gas source (not shown) is connected to the upper electrode 104, and a film forming gas is supplied into the reaction chamber 102 through the upper electrode 104.
Moreover, a high frequency applicator 110 for applying a high frequency is connected to the reaction chamber 102 in the vicinity of the upper electrode 104. Furthermore, an exhaust path 114 for exhausting a gas through a pump 112 is connected to the reaction chamber 102.
In the plasma CVD apparatus 100 thus constituted, for example, monosilane (SiH4), N2O, N2, O2 or Ar in the formation of the film of silicon oxide (SiO2) and monosilane (SiH4), NH3, N2, O2 or Ar in the formation of the film of silicon nitride (Si3N4 or the like) are introduced through the film forming gas supply path 108 and the upper electrode 104 into the reaction chamber 102 maintained in a decompression state of 130 Pa, for example. In this case, a high frequency power of 13.56 MHz is applied between the electrodes 104 and 106 provided opposite to each other in the reaction chamber 102 through the high frequency applicator 110, for example. As a result, a high frequency electric field is generated, electrons are impacted with neutral molecules of a film forming gas in the electric field, and a high frequency plasma is formed so that the film forming gas is dissociated into ions and radicals. By the actions of the ions and radicals, a silicon thin film is formed on a surface of a semiconductor product W such as a silicon wafer which is provided on one of the electrodes (the lower electrode 106).
In such a plasma CVD apparatus 100, at a film forming step, a thin film material such as SiO2 or Si3N4 is also adhered to and deposited on the surfaces of an inner wall, an electrode and the like in the reaction chamber 102 other than the semiconductor product W to be formed by a discharge in the reactive chamber 102 so that a by-product is obtained. When this by-product grows to have a constant thickness, it is peeled by a dead weight, a stress or the like. At the film forming step, consequently, fine particles to be foreign matters are mixed into the semiconductor product so that contamination is caused. For this reason, a thin film of high quality cannot be manufactured so that the disconnection or short-circuit of a semiconductor circuit is caused. In addition, there is a possibility that a yield or the like might be deteriorated.
For this reason, conventionally, a by-product is removed by using, for example, a cleaning gas obtained by adding a fluorocompound such as CF4, C2F6 or COF2, and O2 or the like if necessary in order to remove such a by-product at any time after the film forming step is ended in the plasma CVD apparatus 100.
More specifically, in a method of cleaning the conventional plasma CVD apparatus 100 using such a cleaning gas, a cleaning gas containing a fluorocompound such as CF4, C2F6 or COF2 is accompanied by a gas such as O2 and/or Ar in place of a film forming gas in the film formation and the cleaning gas is introduced into the reaction chamber 102 maintained in a decompression state through the film forming gas supply path 108 and the upper electrode 104 after the film forming step is ended as shown in FIG. 4. In the same manner as the film formation, a high-frequency power is applied between the electrodes 104 and 106 provided opposite to each other in the reaction chamber 102 through the high-frequency applicator 110. As a result, a high frequency electric field is generated, electrons in the electric field are impacted with the neutral molecules of the cleaning gas, and a high frequency plasma is formed so that the cleaning gas is dissociated into ions and radicals. The ions and the radicals react to a by-product such as SiO2 or Si3N4 which is adhered to and deposited on the surfaces of an inner wall, an electrode and the like in the reaction chamber 102 so that the by-product is gasified as SiF4 and is discharged to the outside of the reaction chamber 102 through the exhaust path 114 together with an exhaust gas by the action of the pump 112.
The fluorocompound such as CF4 or C2F6 to be used as the cleaning gas is a stable compound having a long life in the atmospheric air, and furthermore, there is a problem in that a gas discharging process is hard to perform after the cleaning and a disposal cost is increased. Moreover, global warming coefficient potential (values for a cumulative period of 100 years) of CF4, C2F6 and SF6 are 6500, 9200 and 23900 respectively, and they are extremely large. Therefore, an adverse influence thereof on an environment is apprehended.
Moreover, the ratio of a gas discharged to the outside of the reaction chamber 102 through the exhaust path 114 is high, that is, approximately 60% in case of C2F6, for example, and currently, it has an adverse influence on global warming, and a dissociation efficiency is low and a cleaning capability is also low.
In consideration of such actual circumstances, it is an object of the present invention to provide a CVD apparatus capable of executing cleaning in which a by-product such as SiO2 or Si3N4 adhered to and deposited on the surfaces of an inner wall, an electrode and the like in a reaction chamber can efficiently be removed at a film forming step, and furthermore, the amount of a cleaning gas to be discharged is very small, the influence on an environment such as global warming is also lessened, a gas utilization efficiency is also high and a cost can also be reduced, and manufacturing a thin film of high quality. Furthermore, the object of the present invention is also to provide a method of cleaning the CVD apparatus using the CVD apparatus.